Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing better image of the subsurface is an ongoing process.
For a seismic gathering process, as shown in FIG. 1(a), a data acquisition system 10 includes a vessel 12 towing plural streamers 14 that may extend over kilometers behind the vessel. One or more source arrays 16 may be also towed by the vessel 10 or another vessel for generating seismic waves. Conventionally, the source arrays 16 are placed in front of the streamers 14, considering a traveling direction of the vessel 10. The seismic waves generated by the source arrays propagate downward and penetrate the seafloor, eventually being reflected by a reflecting structure (not shown) back to the surface. The reflected seismic waves propagate upwardly and are detected by detectors provided on the streamers 14. This process is generally referred to as “shooting” a particular seafloor area, which area is referred to as a “cell”. However, such a method results in data having poor azimuth distribution.
An improvement to this conventional data acquisition method is the use of wide-azimuth (WAZ) acquisition. In a typical WAZ survey, two streamer vessels and multiple sources are used to cover a large sea area, and all sources and streamers are controlled at a uniform depth throughout the survey. WAZ acquisition provides better illumination of the substructure and, thus, a better final image.
A newer approach, rich-azimuth (RAZ) acquisition, shows promising signs for improving the final image. RAZ acquisition is the combination of multi-azimuth acquisition and wide-azimuth geometry. RAZ acquisition may be implemented by shooting a same cell in three directions, e.g., 30°, 90°, and 150°, each direction being shot in one or two passes. A rose diagram (an example of which will be described below with respect to FIG. 1(b)) for such a rich-azimuth survey shows the benefits of rich-azimuth towed-streamer acquisition,
A seismic data acquisition system 20, shown in FIG. 1(c) depicts a plurality of streamers 24 attached to a streamer vessel 22 which can be used to perform seismic surveys. The streamer vessel bisects two pairs of source vessels 26, 28 with the first pair of source vessels 26 configured 1200 meters cross-line on each side of the streamer vessel 22 and the second pair of source vessels 28 configured 2400 meters cross-line on each side of the streamer vessel 22. It should be noted that the first pair of source vessels 26 are in-line with the streamer vessel 22 and the second pair of source vessels 28 are 8000 meters in-line in front of the streamer vessel 22.
Looking now to FIG. 1(d), a honeycomb pattern 40 for towing the seismic data acquisition system 20 is depicted. The honeycomb pattern 40 is created by towing the prior art seismic data acquisition system 20 in three intersecting directions 42, 44, 46 across the mapping area 48. For the configuration of the seismic data acquisition system 20, the passes across the mapping area 48 in the same direction are separated by 600 meters. It should be noted that the three towing directions 42, 44, 46 are each rotated one hundred twenty degrees from each other and each towing direction does not use an antiparallel acquisition towing pattern. Further, shots are fired, based on a predetermined shot point interval, as the seismic data acquisition system 20 is towed across the mapping area.
Looking now to FIG. 1(b), a Rose diagram 60 of the data collected by the prior art seismic data acquisition system 20 is depicted. The Rose diagram illustrates a 5000 meter distance 62, a 10,000 meter distance 64 and a 15,000 meter distance 66. Continuing with the Rose diagram 60, three azimuthal bands 68, 70, 72 of charted data are shown representing the three towing directions of the seismic data acquisition system 20. Blank regions indicate areas where no data was collected as a result of the seismic data system 20 configuration, tow patterns 42, 44, 46 and the shot point interval. Consequently, it can be seen from the Rose diagram 60 that none of the concentric rings 62, 64, 66 representing a mapping area have complete coverage with collected seismic data.
However, the existing RAZ acquisition techniques can further be improved to increase the illumination and accuracy of the final image by finding an appropriate number and distribution of source arrays and streamer vessels to achieve ultra-long offset together with broadband techniques. Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and improve the accuracy of the final image.